an exclamation used upon hearing or seeing something surprising and exiting, or when describing the thing to others, such as a gorgeous member of the opposite sex or a big party coming up soon; generally accompanied by a snap made with the index finger brought down against the middle finger, like a director snapping, "action!"

You missed a killer show last night, bro! They got down! And, SNAP!, the place was crawling wall to wall with tight booties! It was on!